The Philips Semiconductors data sheet for discrete semiconductors with title “General Magnetic field sensors” of Jun. 12, 1998, further indicated by “File under Discrete Semiconductors, SC17” discloses a magnetic field sensor which comprises four magneto-resistive sensors arranged in a Wheatstone bridge.
The magneto-resistive sensors are composed of a strip of ferromagnetic material called permalloy. The resistance of the magneto-resistive sensor depends on the angle between the current through the strip of ferromagnetic material and the internal magnetization vector. The internal magnetization vector depends on a strength of a component of an external magnetic field applied perpendicular to the current direction in the sensor.
Such a magneto-resistive sensor is by nature bi-stable. A flipping coil is added which generates brief, strong, non-permanent magnetic field pulses of very short duration (a few microseconds) with alternating opposite direction. These alternating magnetic fields, which can easily be generated by simply winding a coil around the sensor, are used to eliminate offset effects. Depending on the current direction of the current pulses through the flipping coil, positive or negative flipping fields are generated in the direction parallel to the current direction. The flipping causes a change in the polarity of the sensor output signal. Essentially, the unknown field in the normal magnetization direction plus the offset is sensed in one half of the sense cycle, while the unknown field in the inverted magnetization direction plus the offset is sensed during the other half of the cycle. This results in two different outputs symmetrically positioned around the offset value. After high pass filtering, rectification and low-pass filtering a single, continuous value free of offset is obtained for the unknown field.
The magneto-resistive sensor comprises four magneto-resistive elements which are arranged in a Wheatstone bridge configuration. A differential amplifier senses the voltage between two connection points of the bridge to provide an output signal representative of the magnetic field measured. It has to be noted that two such bridges are required, each with their own differential amplifier to sense the magnetic field in two different directions.
The data sheet further discloses that an optimal method of compensating for temperature dependent sensitivity differences uses electromagnetic feedback. This method takes into account that the output of the magneto-resistive sensor is independent of the temperature if no external magnetic field is applied. A compensation coil is added to generate a magnetic field which compensates the external field. Thus, the compensation coil generates a magnetic field perpendicular to the magnetic field generated by the flipping coil. If the external magnetic field varies, the sensors output voltage changes too. This voltage change is converted into a current through the compensation coil. The compensation coil produces a magnetic field proportional to the output voltage change. This field has a direction opposite to the direction of the component of the external field which is sensed. Thus, the field of the compensation coil compensates exactly for the external field. The current through the compensation coil is a measure for the sensed external magnetic field.
The data sheet shows in FIG. 26 an integrator IC2B which filters the voltage across the magneto-resistive sensor and which feeds the filtered voltage back to the operational amplifier which amplifies the voltage across the magneto-resistive sensor to obtain an offset compensation.
The prior art magnetic field sensor circuit has the drawback that it does not perform optimally when a disturbing EM (Electro-Magnetic) radiation is present.